The field of the invention relates to sorbents that may be utilized in desulfurization processes. In particular, the field of the invention relates to porous support material that is impregnated with a desulfurization agent that includes zinc oxide and a dopant.
The removal of sulfur compounds from gas and liquid streams is an important part of industrial processes including those utilized in petroleum refining operations. Sulfur is both an environmental hazard when it is a contaminant in fuel for combustion and a poison for several catalytic materials when it is present in electrochemical systems such as fuel cells. Supported metal catalysts are widely used to produce high purity hydrogen in fuel processing systems via such processes as: catalytic reformation, water gas shift (WGS), and preferential oxidation of carbon monoxide (PROX). Supported metal catalysts also are used as electrode materials in fuel cells. The metals in supported metal catalysts generally have a low sulfur tolerance (i.e. 0.1 ppmv sulfur for proton exchange membrane fuel cells (PEMFC) and ˜10 ppmv for solid oxide fuel cells (SOFC)). Unfortunately, typical sulfur concentrations in fuels may be as high as 3000 ppmw.
Metal oxide based sorbents, such as zinc oxide (ZnO) have been developed to remove sulfur compounds, mainly H2S, from gaseous fuels and reformates. Zinc oxide is widely used to remove H2S from gas streams at low temperatures (<500° C.) because of its high equilibrium constant and high sulfur capacity. However, zinc oxide cannot remove H2S below 0.6 ppmv at 400° C. in the presence of 30 vol % water due to equilibrium limitations. Additional desulfurization units may be required which operate at lower temperatures (room temperature to 100° C.) to remove sulfur down to 0.1 ppmv in order to meet polymer electrolyte membrane (PEM) fuel cell requirements. Moreover, during cold startup of a fuel cell system, the fuel processing units experience a temperature change from room temperature to several hundred Celsius. Therefore, a protective sorbent bed may be necessary to remove H2S residuals from the primary desulfurization unit before the unit reaches steady state. Because the reaction between H2S and a metal oxide sorbent at fuel cell stack temperatures are confined only on the outer layer of solid sorbent particles, the sulfur removal capacity at lower temperatures is limited by solid state diffusion and is much lower than that achieved at higher temperatures. The desulfurization performance and capacity of metal oxide based sorbents at low temperatures must be enhanced in order to improve and protect fuel cells against permanent deactivation.
In order to improve desulfurization performance, sorbents with high porosity and small grain sizes are preferred. In this regard, metal oxide sorbents on inert supports are widely used. In supported sorbents, active sorbent substances are supported on secondary oxides to form high surface area and high porosity sorbent particles/extrudates. These secondary compounds (supports), such as silica, mesoporous crystalline material (MCM) silica, titania, and alumina, are mainly inert to sulfur. Supports also may be utilized to enhance the structural stability for the active sorbent and to adhere/hold the sorbent crystallites within the micropores of the support in the absence of grain size and agglomeration and sintering. Supports also serve to stabilize the active metal oxide component against chemical reduction and vaporization. The supported sorbent design also facilitates the incorporation of the sorbent into process system hardware, such as monoliths. Due to the above noted advantages provided by supported sorbents, these systems provide stable performance with extended service lives.
In addition to being combined with inert supports, the active sorbents (e.g., ZnO) can also be mixed with other active metal oxides, which may function as promoter agents for the active sorbent. In particular, copper has been suggested for use in high temperature desulfurization sorbents, where CuO has an extremely high equilibrium sulfidation constant and thus may yield extremely low equilibrium H2S concentrations even at high steam contents and at high temperatures. However, the use of copper in ZnO based sorbent also has drawbacks including oxide reduction, H2S oxidation, and loss in porosity. Some of these limitations might be solved by preparing a well-designed, supported sorbent composition.
Supported CuO—ZnO sorbents have been developed using hydrated Al2O3 as a support. (See U.S. published patent application nos. 2007/0131589 and 2007/0034552). Metal hydroxides also are used for preparing sorbents. (See, e.g., U.S. Pat. No. 6,743,405). However, because these sorbents use hydrated Al2O3 and metal hydroxides, they can be utilized only in low temperature applications (below about 200° C.) and these sorbents cannot be regenerated using relatively high temperatures and stripping air.
Clearly, better sorbent compositions, systems, and methods for removing sulfur compounds are desirable.